Various industrial process employ aqueous solutions that have salt concentration above their natural solubility. Systems using these solutions are often prone to foul which is due to the natural tendency of the dissolved minerals to form precipitates that settle in inopportune places or adhere to surfaces that should stay clean. Process additives have been developed to overcome these problems. Organic compounds containing sulfonate and other functionalities have for example stabilized waters in steam generators. Organo phosphonates and poly carboxylates are known stabilizers for cooling water systems. Polyols with a molecular weight at or below 500 have been used as corrosion and deposition inhibitors in cooling water systems.
The Bayer process is a specific application area where a metal salt is processed under alkaline aqueous conditions. In the typical Bayer process for the production of alumina, bauxite ore is pulverized, slurried in water, and then digested with caustic at elevated temperatures and pressures. The caustic solution dissolves oxides of aluminum, forming an aqueous sodium aluminate solution. The caustic insoluble constituents of bauxite ore are then separated from the aqueous phase containing the dissolved sodium aluminate. Solid alumina trihydrate is precipitated out of the solution and collected as product.
In detail, the pulverized bauxite ore may be fed to a slurry mixer where an aqueous slurry is prepared. The slurry makeup water is typically spent liquor (described below) and added caustic. This bauxite ore slurry is then diluted and passed through a digester or a series of digesters where, under high pressure and temperature, about 98% of the total available alumina is released from the ore as caustic-soluble sodium aluminate. The digested slurry is then cooled from about 200° C. to about 105° C., employing a series of flash tanks wherein heat and condensate are recovered. The aluminate liquor leaving the flashing operation often contains from about 1 to about 20 weight percent solids, saidsolids consist of the insoluble residue that remains after, or is precipitated during, digestion. The coarser solid particles may be removed from the aluminate liquor with a “sand trap” cyclone or other means. The finer solid particles may be separated from the liquor first by settling and then by filtration, if necessary. The slurry of aluminate liquor leaving the flash tanks is diluted by a stream of recycled washer overflow liquor. Any Bayer process slurry taken from the digesters through any subsequent dilution of the slurry, including the flash tanks, but before the primary settler, is referred hereinafter as the primary settler feed.
The primary settler feed is then fed to the center well of the primary settler, where it is treated with a flocculant. As the mud settles, clarified sodium aluminate solution referred to as “green” or “pregnant” liquor, overflows from the primary settling tank and is passed to the subsequent process steps. The aluminate liquor would then generally be further clarified by filtration to give a filtrate with no more than about 10 mg suspended solids per liter of liquor if it contains an unacceptable concentration of suspended solids (at times from about 10 to about 500 mg of suspended solids per liter). During this process, one of the biggest losses in the aluminum refinery plants is auto-precipitation of aluminate across the thickners, washers and the filtration steps. Conventional technology employs the addition of water continuous emulsion polymers or biopolymers for scale control and prevention of this precipitation process. In addition, sodium gluconate, glycerine and other low molecular weight polyols were found to distinctly inhibit the crystallization of aluminum trihydrate. Polyglycerols have been used in the Bayer process to increase the crystal size of aluminum trihydrate at the precipitation stage.
In past work, mono- or polyhydric alcohols with a molecular weight below 500 were also used for the inhibition of corrosion and deposit formation in cooling water systems. It was found that in other industrial water applications low molecular weight (below 500) polypolar organic compound containing hydroxyl and/or primary amino functional groups inhibit amorphous silica scale formation.
Polyols such as glycerol, pentaerythritol, mannitol, and sorbitol as well as polyols based on glycerol have been used in cement slurries as process aids, retarders, and flow improvers.